1. Field of the Invention
The present invention relates to the field of the manufacture of integrated circuits and more particularly to the technique of dry chemical etching of the circuits.
Several types of etching methods enable fine patterning of integrated circuits to be obtained. The most widespread are electron beam etching methods and photoetching techniques using dry chemical etching.
2. Description of the Prior Art
Dry chemical etching methods utilize a reagent element source produced in the form of plasma generated by a high frequency electrical discharge maintained in a vacuum chamber containing a gas or a suitable combination of gases under a low pressure. The discharge provides a plasma producing radicals, ions and electrons which may, with a substrate, provide chemical reactions leading to etching.
Apparatuses which enable this dry chemical etching method to be applied comprise a vacuum chamber with an internal electrode supporting the substrate to be etched, which electrode is maintained at the potential of a high frequency power source. The chamber includes metal surfaces constituting a second electrode that is grounded to the high frequency source.
When the HF source is in operation, the gas contained in the chamber is ionised and becomes a plasma. An ion bombardment of the substrate takes place as a result of the existence of a direct electrical potential of the plasma, this potential being sufficiently positive with respect to the substrate.
The above-described prior art is disclosed in an article of Yasuhiro HORIIKE and Masahiro SHIBAGAKI entitled "A New Chemical Dry Etching" published in Proceedings of the 7th Conference on Solid State Devices, Tokyo, 1975, Supplement to Japanese Journal of Applied Physics, Vol. 15 No. 7, 1975, pages 13-18. The Horiike et al article discloses a method for the selective etching of a material by reagent ions.
Recent experiments show that etching is linked to the ion bombardment of a groove base by energetic particles of some hundreds of electron volts, as shown by M. F. WINTERS in the Journal of Applied Physics 49 (10), 1978.
The inventor's work in the field of electrical discharges has enabled him to demonstrate that if these discharges are applied to chemical etching, it is possible, in certain conditions, to strike the discharge to etch with an anisotropic nature, i.e. a base of the groove is etched much more rapidly that walls of the groove. A paper relating to the general properties of the plasma potential in a high frequency capacitive discharge has been published by the applicant in a report of the Academie des Sciences of Paris Volume 287 dated Dec. 18, 1978.
In a high frequency (HF) capacitive discharge, the plasma is maintained by passing an alternating current from a HF generator across sheaths which adjoin electrodes of the HF generator. This current imposes an alternating potential difference on the sheath. If the frequency of the current is sufficiently low and the power absorbed sufficiently high, the effective value V.sub.A of this alternating potential difference is much greater than the direct potential difference, V.sub.P, coupled to an ambipolar diffusion of the direct discharge at the same electronic temperature.
As a result of this alternating potential difference, plasma electrons may be liberated very rapidly if the direct potential of this plasma is not sufficiently positive to retain these electrons; it has been shown that the direct potential difference, obtained by this effect, and similar to the behavior of a rectifier, is at least equal to V.sub.A .sqroot.2.
In addition, the thickness of the sheath is linked on one hand to this direct potential difference, to the electron density of the plasma and to its electronic temperature, by Child-Langmuir's law; the sheath thickness is, on the other hand, linked to V.sub.A by an equation which expresses the high frequency potential difference across the sheath as the product of the sheath impedance through the high frequency current. From these two equations it is possible to calculate the expression which gives the potential energy of the plasma ions, i.e. the energy which the ions bombarding the substrate would have if they succeeded in passing through the sheath without colliding with the neutral particles.
In these circumstances, the energy W.sub.+ of the ions bombarding the substrate is given by the equation: EQU kT.times.W.sub.+ =1.26 (.omega..sub.p /.omega.).sup.2 W.sub.-( 1)
in which:
k is Boltzmann's constant, PA0 T is the electronic temperature of the discharge; PA0 .omega./2.pi. is the discharge frequency; PA0 .omega..sub.p /2.pi. is the frequency of the plasma linked to the electron density n.sub.e of the plasma by the equation ##EQU1## in which e is the electron charge, m the mass of the electron and .epsilon..sub.o the dielectric permittivity of vacuum; W.sub.- is the energy with which the high frequency source provides the average electron of the discharge between two collisions with neutral particles. PA0 .lambda. is the mean free path of the electrons of the discharge and (P/v) is the high frequency power density, P is the power and v is the volume. In order for these equations to be applicable, it is further necessary for the thickness of the sheath .rho..sub.o of the discharge to be lower than the mean free path .lambda..sub.i of the ions of the discharge, i.e. EQU .rho..sub.o .ltoreq..lambda..sub.i ; .rho..sub.o .ltoreq..lambda./4 (3)
Equation (1) may also be expressed as follows: EQU W.sub.+ =(1.26).sup.2 .epsilon..sup.-2.sub.o e.sup.2 m.sup.-1 (kT).sup.-2 .omega..sup.-4 .lambda..sup.2 (P/v).sup.2 ( 2)
in which:
In effect one has: EQU .lambda..sub.i =.lambda./4 (4)
whence EQU .rho..sub.o =m.sup.-1/2 (W.sub.-).sup.3/2 (kT).sup.-1 .omega..sup.2.sub.p .omega..sup.-3 ( 5)
which may also be expressed: EQU .rho..sub.o =.epsilon..sup.-1 .sub.o e.sup.2 m.sup.-3/4 (P/v).sup.3/2 n.sub.e.sup.-1/2 (kT).sup.-7/4 .lambda..sup.3/2 .omega..sup.-3 ( 6)
and therefore: EQU 4 .epsilon..sub.o.sup.-1 e.sup.2 m.sup.-3/4 (P/v).sup.3/2 n.sub.e.sup.-1/2 (kT).sup.-7/4 .lambda..sup.1/2 .omega..sup.-3 .ltoreq.1 (7)
If the electrical discharge is maintained at the high frequency (.omega./2).pi., this latter condition is often incompatible with the equations which give the particulate energy amounts of the discharge and which are complex equations depending on the specific properties of the gaseous mixtures used.
In effect, examination of Equations (2) and (7) suggests that an excessively high value should not be selected for .omega., to obtain an ion bombardment of sufficient energy (for example 100 eV) on the substrate. Condition (7) can only be obeyed in these circumstances if the value of the electronic density n.sub.e is suitably high and if the power density (P/v) is not too high. Simultaneously obtaining the required values is not in general feasible due to the energy balance equations mentioned above.
The following examples are cases in which the two conditions are not achieved simultaneously: